Best Aperture Radius

Hello All,

I'm a bit confused about the best way to set the aperture radius to do measures. In "The AAVSO
CCD Observing Manual" it is said that the best aperture is 3-4 times the FWHM for increasing precision. However, the standard automatic set in VPhot is 1.5 times the FWHM and is referred as the better radius. I'm been doing some test with different sets and cannot get a clear conclusion. Can anybody help me, please?.

And another question: my urban conditions keep me in a struggle with a very bad seeing and a high light pollution. My FWHM uses to be 4-5 arcsec at best focus (my OTA-sensor resolution is 1.03 arcsec/pixel). I know this is very high and I ought to bin to 2x2 to achieve less resolution, but considering my antiblooming sensor, wouldn't it cause a significant lost in accuracy?